The invention relates to tower cranes, and more particularly to tower cranes of the type having a counterweighted luffing boom. The invention is concerned with stabilization of such a luffing boom when at a steep angle of inclination, to prevent wind forces from pushing the boom backward past the vertical.
During normal operation of a conventional luffing boom tower crane, during which the boom angle, or the angle with the horizontal, is approximately 75.degree. or less, the crane boom is urged in the upwardly pivoting direction by means of cables or lines which are run over a series of sheaves to one or more counterweights. The counterweights tend to lift the boom up or reduce the weight of the boom, to thereby ease the strain on the boom luffing mechanism, so that the boom angle adjustment motor may be smaller than otherwise would have been required.
In tower cranes of typical prior art configuration, when the boom is raised quite steeply, past approximately a 75.degree. angle of inclination, wind forces acting on the boom can be critical. When the boom is facing into the wind at such a steep angle, close to vertical, wind forces tend to blow the boom over backward, which would result in damage to the structure. To avoid this problem, the crane operator was required to limit the boom angle, or simply to restrict crane operation to days when the wind speed was minimal, severely limiting the use of the crane.
It is an object of the present invention to eliminate this problem by arranging a system of counterweight cables and sheaves or pulleys on a tower crane structure such that beyond a certain angle of inclination, the counterweights tend to retard further upward movement of the boom, rather than pulling upward on the boom as in normal operation, when the boom angle is not so steep.